Dim line terminators when they're not hovered

Currently line terminators are slightly brighter than the rest of
the line that's deselected. Fix that.
Test for null when rendering a stack baseline

Returning null in the function we pass into renderLine allows us to
set the d3 .defined() property of the line conditionally, which lets
the baseline handle null data.
Merge pull request #643 from alphagov/refactor/the-matrix-bugfix-stacked-graphs

Stacked graph bugfixes from matrix refactor
Removed drawCursorLine from the app.

This was part of line and is no longer required.
Allow user to hover over parts of the graph with no-data.

This will now show the line (and no data). Before it would select the nearest model.

There are bugs around the line labels that need to be addressed seperatly to this.